The invention relates to a receiver comprising a field-strength detector for measuring the received signal amplitude and a noise reduction circuit connected thereto, via control signal generating device, for reducing audio-frequency noise in the output signal of the receiver, which control signal generating circuit has an attack time constant at a decrease of the received signal amplitude measured by the field-strength detector, and a recovery time constant being greater than the attack time constant.
Such a receiver is disclosed in U.S. Pat. No. 4,157,455.
The noise reduction circuit of the prior art receiver comprises a switchable low-pass filter which precedes a multiplex decoder. In a first switching state of this low-pass filter the whole multiplex signal is passed and in a second switching state only the portion of the multiplex signal over a frequency range up to 10 kHz.
A reduction of the noise in the audio signal to be reproduced is obtained by switching the low-pass filter from the first to the second switching state. The result thereof is that an abrupt change-over is effected from a stereophonic sound reproduction, wherein the impression is obtained that there are several spatially distributed sound sources, to a monophonic sound reproduction, wherein all the sound seems to come from one position. Switching actions of this type from stereo to mono and vice versa suggest spatial leaps and bounds of several sound sources and consequently cause a spatially unsteady sound. Simultaneously with the stereo-to-mono change-over actions, an abrupt suppression of the higher signal frequencies occurs in the monosignal to be reproduced, so that also the tone color of the sound reproduction changes jumpwise.
The switching signal for the control of the switchable low-pass filter is produced by the control signal generating circuit on the basis of the incoming signal field strength measured by the field-strength detector. If a certain threshold voltage is passed during a decrease of the field strength then a change-over from stereo to mono follows almost directly, as the attack time constant of the control signal generating device is substantially zero. At an increase of the field strength, at which said predetermined threshold voltage is exceeded, a switching from mono to stereo is not effected until after said recovery time constant. By choosing a large recovery time constant, it is on the one hand indeed prevented that rapid field strength variations around the threshold voltage result in fast repeating mono-to-stereo change-overs,but on the other hand the duration of the mono-reproduction in response to such field strength variations may become annoying in length.
In addition, at field strength variations at which the threshold voltage is passed, sudden changes occur in the signal-to-noise ratio of the reproduction-audio signal, which, especially when they occur repeatedly, are experienced as particularly annoying.